Envelopes with a transparent panel or window for allowing a visual inspection of the enclosure are well known and widely used.
Window envelopes are manufactured from a web of paper material which is initially cut into blanks having a predetermined shape. Then, a panel is cut from each blank by a panel cutting apparatus to form the window before carrying out the finishing steps of the manufacturing operation such as folding the blanks, gumming, printing and packaging.
Prior art panel cutting apparatuses use a rotary cutter comprising a cutting tool in the form of a massive cutting die and a rotary die holder to bring the cutting die successively in engagement with the envelope blanks which advance in serial order on a conveyor.
The die holder is a metallic cylinder mounted for rotation on a drive shaft synchronized with the conveyor so that, at each revolution of the die holder, the cutting die engages a different envelope blank.
The cutting die and the die holder are provided with small passages in register with an orifice on the drive shaft which communicates through a cavity, formed in the drive shaft, with a source of vacuum. The purpose of this arrangement is to allow a localized vacuum zone to be formed in the vicinity of the cutting die in order to retain to the cutting die and carry away the panels which are cut out from the blanks.
Typically, the drive shaft is provided with a plurality of orifices at different locations thereon to accept die holders of various types and sizes. The vacuum supply to each orifice is controlled by means of valves by the operator of the panel cutting apparatus who selects the orifice to which vacuum is to be applied and shuts off the vacuum to all the other orifices.
On the drive shaft on which is mounted the die holder are also mounted a pair of rubber rollers engaging the blanks while the cutting die cuts out the panels to maintain registration accuracy.
The prior art panel cutting apparatuses suffer from numerous drawbacks, the most acute being the lack of adjustability of the apparatus to cut out panels of different sizes as well as at different locations on the blank. In most cases, from one production run to another, it is necessary to use different cutting dies and, consequently, different die holders. The cutting dies and the die holders are expensive parts since they require an extensive machining work to be fabricated. Also, the installation of the cutting die and the die holder on the panel cutting apparatus requires long and fastidious adjustments resulting in a loss of production time.